OF THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 349 



ditions. — If we accept, merely for the sake of the argument, the 

 marine origin of the Tanganyika halolimnic gasteropods, and 

 still further if we suppose that they are derived from certain 

 indicated Jurassic forms, it becomes a question when and where 

 the transference from marine to fresh-water conditions was 

 effected ; in other words, when and wliere did their ancestors 

 cease to be marine molluscs and become fresh- water ones. I 

 have already said that it is to the immense Congo basin that 

 we must look for any indications on the subject ; but before 

 making any attempt in this direction it may be as well to point 

 out the difficulty in supposing that this transference was 

 effected anywhere in the Tanganyika region itself. If sucli a 

 transference ever took place we should seek for it rather in 

 some region where Jurassic beds are known to occur, or at 

 least in their neighbourhood, unless we leave everything to mere 

 conjecture. Again the question when, i.e. to say, at what 

 geological period, did the transference take place is equally 

 important. The original Jurassic stock of our hypotliesis must 

 have existed as Cretaceous molluscs during the Cretaceous 

 period and as Tertiary molluscs during the Tertiary period. It 

 may be argued that these considerations are in favour of an 

 early separation from a marine area, since fresh-water conditions 

 are held to be conservative of form, and consequently the more 

 remote in time the transference took place the less likelihood of 

 change in the morphology of the shells. 



Undoubtedly, in the long run, these questions of when and 

 where, which I have put before the members of the Institute, 

 must be determined by geological and above all by palseontolo- 

 gical considerations. The nearest known Jurassic fauna of any 

 importance which has hitherto been described is that of north- 

 west Madagascar distant in an air-line from the south end of 

 Lake Tanganyika about 1,400 miles, and almost on the same 

 parallel of south latitude. The improbability that the halo- 

 limnic stock was derived from this source has already been 

 indicated, owing to the physical structure of East Equatorial 

 Africa, which we shall presently proceed to study. It is on 

 the whole a fortunate circumstance for the hypothesis of a 

 Jurassic origin for the Tanganyika stock that this is the case, 

 for in ihese Jurassic deposits, which would have the advantage 

 of being under the same conditions with respect to latitude and 

 presumably in the same zoological province as the area of 

 Tanganyika in Jurassic times, there is not a single genus of 

 gasteropods wliich has any especial resemblance to the halolimnic 

 gasteropods of Tanganyika. See ante, p. 347. 



